1. Field of the Invention
This invention relates to a display control device for a display apparatus, which may be advantageously employed as a display means in a computer system such as a personal computer, a word processor or the like.
2. Description of the Prior Art.
A cathode-ray tube or a liquid crystal display panel is used as a display means for a personal computer, a word processor or the like. The screen of such a display means is composed of a plurality of picture elements (pixels) arranged in a matrix to form visual display areas. On the other hand, image data to be displayed on such a display means are stored in an image memory (video random access memory which is hereinafter referred to as "VRAM"), and are controlled by an application software. The area in the VRAM managed by an application software is known as the world coordinate area, and the display area formed on the screen of the display means as the screen coordinate area.
When the world coordinate area has the same size as the screen coordinate area, no problem arises. But, when the screen coordinate area is smaller than the world coordinate area, for example, when the screen coordinate area is made up of pixels of 640 dots per horizontal line (width) and 480 dots per vertical line (height) while the world coordinate are consists of 720-dot horizontal lines and 480- dot vertical lines, the problem arises in that the display area on the screen cannot cover the full width of the world coordinate area in the horizontal direction.
To overcome the above-mentioned problem, there has previously been employed either of the following two techniques.
One is a window display technique in which pixel data representing 640 dots shown in (1) of FIG. 6 are extracted from a world coordinate area A1 to be displayed on a screen P1, P2 or P3 which corresponds to a screen coordinate area A2. For example, as shown in (2) of FIG. 6, pixel data representing 640 dots beginning from point a to point e in the horizontal direction is extracted from the world coordinate areas A1 to be displayed as the screen P1. In this case, the image portion formed on 40 dots at each side of the world coordinate area A1 is not displayed. In the case shown in (3) of FIG. 6, pixel data representing 640 dots beginning from point b (which is the starting point of the horizontal line) to point d are extracted from the world coordinate area A1 to be displayed as the screen P2. In this case, the image portion formed on 80 dots at the right side of the world coordinate area A1 is not displayed. In the case shown in (4) of FIG. 6, pixel data representing 640 dots beginning from point c to the right edge of the world coordinate area A1 are extracted to be displayed as the screen P3, so that the image portion formed on 80 dots at the left side of the world coordinate area is not displayed. By sequentially displaying the screens P1, P2 and P3, each having a missing image portion, in a suitable manner, the whole image can be displayed on the screen. In the prior art, the addresss change for reading out the world coordinate area A1 in the VRAM mentioned above is executed by instructions from the software.
The other prior art technique is a so-called reduced display technique in which, as shown in FIG. 7, the ratio of 640:720 (=8:9), which is the ratio of the number of dots on the screen coordinates area A2 to that on the world coordinate areas A1 in the horizontal direction, is taken as the reduction ratio for the pixel data in the horizontal direction to be displayed on the screen. More specifically, data representing nine pixels are extracted from the VRAM to be converted to data representing eight pixels, and the converted data are rewritten into the VRAM. Then the converted data are read out from the VRAM to be displayed, so that the image reduced to 8:9 in the horizontal direction is displaced on the screen.
The above-mentioned window display technique has the drawbacks that the world corrdinate area A1 cannot be displayed at one time, and that the address for reading data from a VRAM has to be forcibly changed to the one different from that specified by the original software, resulting in the loss of software compatibility with regard to display control.
On the other hand, with the reduced display technique, because pixel data must be reduced with a predetermined reduction ratio (e.g., 8:9) in the horizontal direction, a prolonged period of time is required for data processing, resulting in that it is difficult to conduct the realtime display. Moreover, since the process involves the modification of the contents of the VRAM, the original data will be lost. The once lost data cannot be restored. Furthermore, if data are reduced simply to 8:9, one data is lost for every nine data, which leads to the distortion of the resulting image although the entire area can be displayed. Also, with the software available in the prior art, it is not possible to accomplish the reduction ratio of 8:9, and in practice, a reciprocal number of a power of 2 such as 1/2, 1/4, etc. has been used to determine the reduction ratio. This results in the loss of the compatibility of softwares because of the intervention of a software required for such reduction processing.